JP5595913B2 - Laser lap welding method of galvanized steel sheet - Google Patents

Laser lap welding method of galvanized steel sheet Download PDF

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JP5595913B2
JP5595913B2 JP2010519799A JP2010519799A JP5595913B2 JP 5595913 B2 JP5595913 B2 JP 5595913B2 JP 2010519799 A JP2010519799 A JP 2010519799A JP 2010519799 A JP2010519799 A JP 2010519799A JP 5595913 B2 JP5595913 B2 JP 5595913B2
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laser
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galvanized
galvanized steel
irradiation
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JPWO2010005025A1 (en
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聖二 片山
洋介 川人
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スズキ株式会社
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Priority to PCT/JP2009/062443 priority patent/WO2010005025A1/en
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/14Working by laser beam, e.g. welding, cutting or boring using a fluid stream, e.g. a jet of gas, in conjunction with the laser beam; Nozzles therefor
    • B23K26/142Working by laser beam, e.g. welding, cutting or boring using a fluid stream, e.g. a jet of gas, in conjunction with the laser beam; Nozzles therefor for the removal of by-products
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/20Bonding
    • B23K26/21Bonding by welding
    • B23K26/24Seam welding
    • B23K26/244Overlap seam welding
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00Articles made by soldering, welding or cutting
    • B23K2101/18Sheet panels
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00Articles made by soldering, welding or cutting
    • B23K2101/34Coated articles, e.g. plated or painted; Surface treated articles
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00Materials to be soldered, welded or cut
    • B23K2103/02Iron or ferrous alloys
    • B23K2103/04Steel or steel alloys
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00Materials to be soldered, welded or cut
    • B23K2103/08Non-ferrous metals or alloys

Description

  The present invention relates to a laser lap welding method for galvanized steel sheets, and more particularly to a laser lap welding method for galvanized steel sheets when a large amount of galvanized steel sheets are lap welded with a laser in the automobile industry or the like.

  In the automobile industry, not only excellent corrosion resistance but also high specific strength and low cost, galvanized steel sheets (hereinafter referred to as “galvanized steel sheets”) are widely used. At this time, depending on the application, it is necessary to weld two galvanized steel sheets, but laser welding with superior characteristics such as high accuracy, high quality and high-speed processing compared to spot welding etc. It is preferred to do this.

  When two galvanized steel sheets are overlapped and welded by laser (hereinafter, such welding is also referred to as “laser lap welding”), for example, the galvanized steel sheets are stacked one above the other with the galvanized layers facing each other, Laser light from a carbon dioxide laser or YAG laser is irradiated to melt the upper and lower galvanized steel sheets and join them.

  In order to achieve good bonding, the iron layers of the upper and lower galvanized steel sheets need to be mixed with each other. The melting point of zinc is about 420 ° C., the boiling point is 907 ° C., and the melting point of iron is about 1535 ° C. It is considerably low compared. For this reason, simply overlapping the galvanized steel sheets so that the galvanized layers face each other, and simply irradiating the welding spot with laser in that state, blows away the surrounding molten metal when the zinc of the galvanized layer evaporates, Hole defects (a kind of welding defects) such as pits, porosity, and worm holes due to remaining as bubbles in the molten metal are formed.

  As a countermeasure, a gap of about 0.1 mm for escaping zinc vapor is provided between the galvanized steel sheets to be laser lap welded. In this state, for example, a YAG laser with an output of about 4 kW is used for 3 to 4 m / Laser lap welding is performed at a speed of about min.

  Further, in order to efficiently form the gap, the vicinity of the portion where one of the galvanized steel sheets is subjected to laser lap welding is bent in advance by laser irradiation, and then, for example, 5 m / min with a power of about 6 kW. It has been proposed to perform laser lap welding at a moderate speed (Claim 1, Paragraph 0026 of Patent Document 1).

  In addition, when laser lap welding of three or more galvanized steel plates, for example, a CW wave with an output of about 2.5 kW is irradiated with a gap of about 10% of the plate thickness between each galvanized steel plate. It has been proposed to weld at a speed of about 1.5 m / min. This is also based on the same idea as described above in which a gap through which zinc vapor escapes is formed between the steel plates to be welded (claim 1, claim 2, paragraph 0019, paragraph 0021 of Patent Document 2).

JP 2005-144504 A JP 2005-262226 A

  However, forming a gap of about 0.1 mm between the galvanized steel plates stacked one above the other takes time and is difficult to manage the process. For example, in the invention described in Patent Document 1, it is necessary to perform laser irradiation twice. In particular, in the automobile industry, there are a large amount of galvanized steel sheets to be processed and the thickness of the steel sheet is about 1 mm, so that it takes more time and management of the process becomes difficult.

  As described above, since laser welding has technically superior characteristics compared to spot welding, the properties of the galvanized steel sheet that has been laser lap welded are also excellent. However, laser lap welding of galvanized steel sheets has not been widely adopted because the initial introduction cost is high and there are difficulties as described above.

  For this reason, in order to be able to make use of the superior characteristics of laser lap welding for galvanized steel sheets, galvanization can be performed on a large number of galvanized steel sheets without much time and process management. Development of a laser lap welding method for steel sheets has been desired.

In order to achieve the above object, the present invention provides:
Two steel plates, at least one of which is a galvanized steel plate, are overlapped with the galvanized layer as a joining surface, and laser welding is performed on one surface of the two steel plates in the superposition region to perform lap welding. In the laser lap welding method of the galvanized steel sheet to be performed,
By irradiating the laser while traveling at a power of 5 kW or more, a power density of 10 kW / mm 2 or more in terms of Gaussian shape , and a constant traveling speed of 10 m / min or more, in the molten pool extending backward from the laser irradiation position, At least partially and temporarily on the steel plate on the surface side, an elongated hole whose length in the laser traveling direction is at least twice as long as the width is generated, and the metal vapor generated by laser irradiation is rearward in the laser traveling direction from the elongated hole. And it welds, discharging | emitting to the laser irradiation source side.

  By the above method, zinc vapor generated by the evaporation of zinc on the overlap surface escapes from the elongated hole generated in the molten pool without adversely affecting the molten pool, and as a result, excellent laser lap welding without hole defects is achieved. It becomes possible.

  In other words, by adjusting the irradiation method such as laser power, laser irradiation power density, irradiation spot diameter, defocus amount, traveling speed, etc., welding is performed at high speed and high energy density, so that the laser irradiation position and key The holes (melting and other dents that occur when the metal evaporates) deviate, and the metal evaporation concentrates at the front end of the keyhole in the direction of laser irradiation, and the metal vapor is behind the direction of laser irradiation and the laser irradiation source. The keyhole becomes a long and narrow hole, and zinc vapor escapes mainly from the tip and its periphery, and from the side wall. Therefore, when escaping, the molten metal in the molten pool on the laser irradiation source side (upper side if the galvanized steel sheets are stacked one above the other) was blown away, or zinc vapor remained in the molten pool It is not to be. As a result, laser lap welding of large quantities of galvanized steel sheets can be performed without much effort, process management can be easily performed, and laser lap welding with excellent technical characteristics can be widely applied to lap welding of galvanized steel sheets. Be available.

  In the present invention, of the two steel plates, a case where a galvanized layer is provided on one side or both sides of a steel plate on the laser irradiation surface side, and a galvanized layer is not provided on the other steel plate, a laser irradiation surface A case where a galvanized layer is provided on one or both sides of the steel plate on the side and a galvanized layer is provided on one or both sides of the other steel plate, and a galvanized layer is provided on the steel plate on the laser irradiation source side All cases are included, in which the other steel plate is provided with a galvanized layer on one side or both sides.

  And “superimposing a galvanized layer as a contact surface” refers to superimposing such that there is a zinc layer on the contact surface of the stacked steel plates, specifically, a steel plate on the laser sealing surface side and / or It means that two steel plates are overlapped so that at least one of the galvanized layers provided on the other steel plate becomes a contact surface.

  Here, the “galvanized steel sheet” is mainly used for automobiles, the plate thickness is 0.5 to 2 mm, the thickness of the galvanized layer is 4 to 12 μm, and the steel is mild steel, alloy Steel, high-tensile steel, etc. Zinc plating is not limited to plating with pure zinc, but may be any metal plating that uses zinc as a main material as long as the effect of the present invention is exhibited.

  The “elongated hole” refers to an elongated keyhole of the molten pool that reaches the galvanized layer, and “elongated” means that the length in the laser traveling direction is longer than the width in the direction perpendicular thereto, preferably The length is at least twice the width, more preferably at least 3 times, and even more preferably at least 4 times.

  “Adjust laser power, laser irradiation power density, and irradiation method” means to select an appropriate type of laser device or an appropriate power laser device, or to determine the laser irradiation spot diameter and defocus amount. And adjusting the running speed of laser irradiation. As a result, when the elongated hole is formed in the molten pool made of molten steel formed by laser irradiation, and the zinc of the plating layer located thereunder is vaporized, it should be Do not blow away the molten metal.

  In addition to the above, it is preferable to use an inert gas for welding the steel plate with a laser. This is the same as ordinary laser welding, and a gas such as argon, helium, nitrogen, carbon dioxide gas is sprayed from the running direction side to the welding location at a flow rate of about 20 to 80 L / min. Furthermore, in the case of a galvanized steel sheet for automobiles, it is preferable from the viewpoint of cost, quality, and the like that argon is about 30 L / min. However, the present invention does not exclude laser remote welding that does not use any gas.

  Even when three or more steel plates are lap-welded, they are not excluded from the present invention as long as the welding method of two steel plates satisfies the above requirements.

  The method of the present invention is particularly preferably carried out when the thickness of the steel plate on the surface side is 0.5 to 2 mm and the thickness of the galvanized layer is 4 to 12 μm among the two steel plates. it can.

  However, the individual welding conditions are specifically determined while adjusting for other conditions according to the thickness of the steel sheet on the irradiation surface side to be welded. For example, in the case where the plate thickness is large, if other conditions are the same, in order to make the “elongate hole” deep, not only to deepen the molten pool (melting), The laser power density is increased in proportion to the plate thickness. Even when the plate thickness is the same, when the power density of laser irradiation is increased, the traveling speed of laser irradiation is increased in proportion thereto.

  In the present invention, the “steel plate” of the “laser-irradiated surface side steel plate” includes a steel plate not provided with a galvanized layer, in addition to a galvanized steel plate on which one side or both sides are galvanized. That is, the “galvanized steel sheet” may be on both sides when it is on the laser irradiation surface side, on the opposite side of the laser irradiation surface. The “thickness of the galvanized layer” refers to the thickness of the galvanized layer of one steel plate.

  With regard to the “laser power”, it is preferable that the output is large in terms of increasing the traveling speed of irradiation while appropriately forming an elongated hole through which zinc vapor escapes from the tip. However, if it is excessive, such as 20 kW or more, it is not preferable from the viewpoint of increasing the equipment cost.

  In the method of the present invention, a laser energy having a higher density than that conventionally used is irradiated at a welding point at a higher speed than before, so that the plate thickness and the thickness of the galvanized layer as defined above are reduced. In laser lap welding of the galvanized steel sheet, it is possible to perform more sound welding. The metal on the irradiation surface center of the welded steel plate, that is, the center of the focal point and on the line along the traveling direction, has a longer irradiation time than the side of the irradiation surface (the direction orthogonal to the traveling direction), so it vaporizes first. In addition, the surrounding molten metal escapes backward and upward while pushing the molten metal laterally and backward in the running direction. For this reason, an elongated recess (elongated keyhole) is suddenly formed in the molten pool. Further, the elongated recess reaches a depth reaching the galvanized layer, and vaporized zinc escapes backward and upward from the tip of the elongated recess. As a result, sound laser lap welding can be performed at high speed.

In the method of the present invention, the power density of the laser irradiation is more preferably 15 kW / mm 2 or more in terms of Gaussian shape.

Here, the “power density of laser irradiation” is preferably larger to some extent from the viewpoint of deeply welding two steel sheets and performing firm lap welding, but the power density of laser irradiation is, for example, 30 kW / It is not preferable from the aspect of energy cost (efficiency) to make it larger than a certain value such as mm 2 or more. Further, even when the plate thickness, particularly the plate thickness on the laser irradiation side, is thin, excess energy is not preferable.

  In the method of the present invention, the laser irradiation spot diameter is 0.1 to 2 mm, the focal length of the lens of the laser irradiation apparatus is 100 to 1500 mm, and the laser defocus amount is 0 to 30 mm. Is preferred.

  If the laser irradiation spot diameter (focus size) and the defocus amount are optimized, the welding location and the temperature conditions in the vicinity thereof contribute to sound laser lap welding. For this reason, for example, the molten pool can be formed while maintaining an appropriate traveling speed, and the depth and width thereof are also appropriate, so that the zinc vapor generated from the galvanized layer can easily escape, and as a result, a more sound laser Lap welding will be obtained.

In the method of the present invention,
Of the two steel plates, the thickness of the steel plate on the laser irradiation surface side is 0.7 to 1.2 mm,
The thickness of the galvanized layer of the galvanized steel sheet is 5 to 10 μm,
The laser power is 8 kW or more,
The power density of the laser irradiation is 18 kW / mm 2 or more in terms of Gaussian shape,
The irradiation spot diameter of the laser is 0.1 to 1.2 mm,
The focal length of the lens of the apparatus for irradiating the laser is 150 to 1200 mm,
The defocus amount of the laser is preferably 0 to 20 mm.

  This prescribes the most preferable range of other requirements when a steel plate having a thickness of 0.7 to 1.2 mm is used for the laser irradiation surface side of the two steel plates.

  Of the two steel plates, the thickness of the steel plate on the side opposite to the laser irradiation side is not particularly limited. In other words, the steel plate on the laser irradiation side vaporizes to generate a laser-induced plume, and an elongated keyhole is formed, from which the zinc at the contact portion of both steel plates evaporates without blowing off the molten metal on the laser irradiation side Does not seem to have a big impact on For this reason, if the steel plate on the laser irradiation side is 0.7 to 1.2 mm, the present invention in which preferable welding conditions are set can be applied as it is unless the plate thickness of the steel plate on the opposite side changes significantly. It seems that excellent laser lap welding is possible.

As for “power density of laser irradiation”, in the laser lap welding of the galvanized steel sheet having the thickness of the steel sheet and the thickness of the galvanized layer, 24 kW / mm 2 or less in terms of Gaussian shape for the above reason. It is preferable that it is 18-22 kW / mm < 2 > practically. Further, the laser power is also preferably 20 kW or less, and more preferably about 5 to 15 kW in total.

  For example, when the laser irradiation power density and laser power are within the above ranges, the laser irradiation spot diameter is about 0.3 mm, and the defocus amount is about 15 mm, the laser irradiation traveling speed is the laser power. If it is 5 kW, it will be about 10 m / min, if it is 7 kW, it will be about 11 m / min, if it is 10 kW, it will be about 12 mm / min, and if it is 13 kW, it will be about 15 m / min.

  The “steel plate”, “steel plate on which galvanization has been performed”, and “thickness of the galvanized layer” of “the steel plate on the laser irradiation surface side” are the same as described above.

  In addition, regarding the influence of the “thickness of the galvanized layer” on the welding conditions, the amount of zinc being plated itself is quantitatively smaller than that of the steel plate, and the melting point of the steel is extremely low compared to the boiling point of zinc. In terms of the time required for evaporation of zinc plating due to irradiation with intense laser energy, zinc plating may evaporate from the tip of the elongated hole without blowing off the molten metal in the molten pool, etc. The influence of the layer thickness is relatively small, so it seems that there is no need to greatly change the welding conditions depending on the thickness of the galvanized layer.

  In the method of the present invention, it is preferable that the laser is a fiber laser and uses a wavelength of 1000 nm band.

  Uses a fiber laser (fiber transmission laser, welding fiber laser, etc.) and has a wavelength of 1000 nm (800-1500 nm for fiber transmission lasers, 1000-1100 nm for welding fiber lasers) suitable for fiber lasers. By using this laser, welding becomes easy. In particular, it becomes easy to set the irradiation position, adjust the focus, and adjust the traveling speed.

  As described above, according to the present invention, laser lap welding of a large amount of galvanized steel sheets can be carried out without the need for labor, and the process can be easily managed, and laser lap welding having excellent technical features. Can be widely used for lap welding of galvanized steel sheets.

It is a perspective view which shows the laser lap welding of the galvanized steel plate as one Example of this invention. FIG. 2 is a perspective view conceptually showing the behavior of a weld metal melt and steam during welding shown in FIG. 1. It is sectional drawing along the running direction which shows the welding location at the time of the welding shown in FIG. 1 notionally. It is the figure seen from the upper side which shows the welding location at the time of the welding shown in FIG. 1 notionally. It is a microscope picture of the cross section of the direction orthogonal to the running direction of the laser irradiation of a laser lap welding location.

DESCRIPTION OF SYMBOLS 10 Fiber 11 Lens 17 Laser beam 18 Ray focus 19 Laser irradiation spot 20, 21 Galvanized steel plate 30 Nozzle 35, 36 Holding jig 40 Melting point tip 41 Laser induction plume 42 Elongated holes 45, 46 On both sides of elongated holes The resulting weld pool 47 The weld pool 48 behind the elongated hole Weld bead

  Hereinafter, the present invention will be described based on the best mode. Note that the present invention is not limited to the following embodiments. Various modifications can be made to the following embodiments within the same and equivalent scope as the present invention.

    In FIG. 1, a mode that the laser lap welding of the galvanized steel plate as one Example of this invention is performed is shown notionally. In FIG. 1, 10 is a fiber of a laser transmitter, 11 is a lens, 20 and 21 are galvanized steel sheets stacked on top (20) and bottom (21), and 30 is a nozzle for blowing an inert gas. And 35 and 36 are holding jigs for the galvanized copper plate. Reference numeral 17 denotes a laser beam, 18 denotes a focal point of the laser beam, an arrow in a beam indicating the laser beam 17 indicates a laser irradiation direction, and 19 denotes a laser irradiation formed on the galvanized steel sheet 20. Reference numeral 48 denotes a weld bead. Also, the thick arrow is the direction of laser irradiation (the direction in which welding is performed), and the thin arrow at the tip of the nozzle 30 is the flow of inert gas. Furthermore, d represents the defocus amount of laser irradiation.

  The two galvanized steel sheets 20 and 21 are for ordinary automobiles, and 5 μm galvanizing is applied to both surfaces of a 1 mm thick steel sheet. Prior to laser lap welding, these two galvanized steel sheets 20 and 21 are wiped up and down with ethanol after wiping off oil and dirt on the surface thereof, and both ends thereof are fixed by pressing jigs 35 and 36. For this reason, the upper and lower galvanized steel sheets 20 and 21 are in close contact with the galvanized layer as the contact portion.

As the laser oscillator, a laser oscillator having an output of 10 kW and a wavelength of 1070 nm manufactured by IPG Photonics Japan Co., Ltd. was used. The laser beam 17 emitted from the fiber 10 of the laser transmitter travels in the welding direction (right side in the figure) while irradiating the laser beam 17 from the direction orthogonal to the surface of the welding surface (galvanized steel sheet 20). To do. At the time of welding, the laser beam 17 is focused at 15 mm before the welding surface (just above the figure) (defocus amount = 15 mm), and the lens 11 is adjusted so that the spot diameter becomes 1.1 mm, and at 12 m / min. I drove it. Accordingly, the power density of the welded portion is 21 kW / mm 2 assuming that the cross-sectional strength distribution is Gaussian. Note that the focal length of the lens 11 in this embodiment is 250 mm.

  The inert gas was Ar, which is an inexpensive inert gas, and was blown at a flow rate of 30 L / min using a nozzle 30 with a diameter of 16 mm from the front in the laser beam traveling direction and 45 degrees above.

  2 to 4 conceptually show the behavior of the weld metal melting wave and steam during the welding. FIG. 2 shows the welded portion as viewed from obliquely above, FIG. 3 shows a cross section along the traveling direction of the welded portion, and FIG. 4 shows the welded portion seen from above. In these drawings, reference numeral 40 denotes a melting point tip, reference numeral 41 denotes a laser-induced plume, reference numeral 42 denotes an elongated hole (elongated keyhole) generated by ejected metal vapor, and 45 and 46 are on both sides of the elongated hole 42. The resulting molten pool 47 is a molten pool behind the elongated hole. Also in these drawings, the thick arrow indicates the traveling direction of laser irradiation. Furthermore, arrows with thick broken lines indicate the flow of metal vapor.

  Although the upper and lower galvanized steel sheets 20 and 21 are melted by laser irradiation, since the irradiation energy density is large, the melting point tip 40 is deeply melted deeply at the rear side in the running direction, and a part of the metal rapidly from the surface. The metal vapor (laser-induced plume) generated by the evaporation further evaporates and pushes the liquid metal in the surroundings and upper part (laser irradiation side) backward and laterally in the direction of travel, slightly after the irradiation point (with the direction of travel). It is ejected from the opposite side (left side in the figure) toward the rear and the upper side (laser irradiation side).

  The laser-induced plume 41 is ejected in the above-mentioned direction because not only the laser beam is irradiated for the longest time in the vicinity of the center line in the traveling direction of the irradiated portion, and the power density of the laser beam is high, but also the traveling direction of irradiation. This is because there is a solid metal layer that is not yet melted on the side and the irradiation direction side (downward in FIGS. 2 and 3) and on both sides in the traveling direction of the irradiation point (vertical direction in FIG. 4). For this reason, the laser-induced plume 41 occurs along the center line in the traveling direction of the irradiated part. As a result, a laser-induced plume 41 occurs behind the laser irradiation position and along the center line in the traveling direction of irradiation. As a result, an elongated hole 42 is formed in the traveling direction in which no molten metal exists at that position. Furthermore, elongated molten pools 45 and 46 are formed on both sides of the elongated hole 42 in the traveling direction, and further flows in a direction opposite to the traveling direction due to the metal vapor pressure, and merges on the rear side of the elongated hole 42 in the traveling direction. Become. In this example, it was recognized that an elongated hole (elongated keyhole) having a width of about 1 mm and a length of about 3 mm was formed.

  In the present invention, not only simply an elongated hole is formed, but also zinc vapor is spouted upward and rearward as the laser-induced plume 41 or a part thereof from the tip or the periphery of the formed elongated hole. No molten metal is blown off or only slightly blown, and no zinc vapor remains in the molten pool.

  As described above, the melting point (419.5 ° C.) and the boiling point (907 ° C.) of zinc are not only much lower than the melting point of iron (1535 ° C.), but also the heat of fusion and the heat of vaporization (7. 322 kJ / mol, 115.3 kJ / mol) (iron, which is the main material of the steel sheet, is 13.8 kJ / mol, 349.6 kJ / mol, respectively. However, the above four numerical values are the additives in zinc and steel sheet. In fact, it is slightly different due to the influence of the formulation). For this reason, if the amount of heat transfer from the steel plate located on the laser irradiation side is large, zinc is instantaneously melted and vaporized, and a large amount of the generated zinc vapor blows away the molten metal above it.

  However, iron has a lower thermal conductivity than copper or the like, and the molten liquid has a lower thermal conductivity than that of solid, and as mentioned above, the heat of vaporization of zinc is small. On the other hand, the density of irradiation energy is large. Furthermore, the focal length, defocus amount, etc. of the laser irradiation apparatus are also appropriate. As a result, the steel is melted and evaporated sequentially from the surface on the irradiated side of the galvanized horizontal plate, and then the zinc on the contact surface of the galvanized steel plates 20 and 21 at the irradiated portion is rapidly melted with the energy by laser irradiation. Since it is vaporized and ejected from the tip or the periphery of the elongated hole, good lap welding is performed.

  The sample welded in this example was cut in a direction orthogonal to the traveling direction of laser irradiation, and the cross section was observed with a microscope to examine the welding situation. FIG. 5 is a micrograph of the cross section. As shown in FIG. 5, no welding defects are observed in the cross section, and it can be seen that the laser lap welding of a good galvanized steel sheet can be performed without providing a gap between the two steel sheets by the method of the present invention.

Claims (6)

  1. Two steel plates, at least one of which is a galvanized steel plate, are overlapped with the galvanized layer as a joining surface, and laser welding is performed on one surface of the two steel plates in the superposition region to perform lap welding. In the laser lap welding method of the galvanized steel sheet to be performed,
    By irradiating the laser while traveling at a power of 5 kW or more, a power density of 10 kW / mm 2 or more in terms of Gaussian shape , and a constant traveling speed of 10 m / min or more, in the molten pool extending backward from the laser irradiation position, At least partially and temporarily on the steel plate on the surface side, an elongated hole whose length in the laser traveling direction is at least twice as long as the width is generated, and the metal vapor generated by laser irradiation is rearward in the laser traveling direction from the elongated hole. And the laser lap welding method of the galvanized steel sheet characterized by welding while discharging to the laser irradiation source side.
  2.   2. The zinc according to claim 1, wherein a thickness of the steel plate on the surface side of the two steel plates is 0.5 to 2 mm, and a thickness of the galvanized layer is 4 to 12 μm. Laser lap welding method for plated steel sheet.
  3. The laser lap welding method for a galvanized steel sheet according to claim 1 , wherein the power density of the laser irradiation is 15 kW / mm 2 or more in terms of Gaussian shape.
  4. An irradiation spot diameter of the laser is 0.1 to 2 mm, a lens focal length of the laser irradiation apparatus is 100 to 1500 mm, and a defocus amount of the laser is 0 to 30 mm. Item 4. A laser lap welding method for a galvanized steel sheet according to Item 3 .
  5. Of the two steel plates, the thickness of the steel plate on the laser irradiation surface side is 0.7 to 1.2 mm,
    The thickness of the galvanized layer of the galvanized steel sheet is 5 to 10 μm,
    The laser power is 8 kW or more,
    The power density of the laser irradiation is 18 kW / mm 2 or more in terms of Gaussian shape,
    The irradiation spot diameter of the laser is 0.1 to 1.2 mm,
    The focal length of the lens of the apparatus for irradiating the laser is 150 to 1200 mm,
    2. The laser lap welding method for galvanized steel sheets according to claim 1, wherein a defocus amount of the laser is 0 to 20 mm.
  6. The laser lap welding method for a galvanized steel sheet according to any one of claims 1 to 5 , wherein the laser is a fiber laser and uses a wavelength in a 1000 nm band.
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